The present invention relates generally to methods for processing titanium alloys in the fabrication of powder metallurgy (PM) titanium alloy articles and more particularly to a method for producing substantially full density titanium aluminide foil.
Titanium aluminide based matrix composites have potential significant high temperature applications in the temperature range to about 1500.degree. F. for Ti.sub.3 Al (alpha-2) based composites and to about 1800.degree. F. for TiAl (gamma) based composites mainly because of their characteristic low density, high temperature strength and modulus, and oxidation resistance. Conventional methods for producing titanium alloy foil which include vacuum annealed cold rolling to a desired thin gauge in successive cycles generally cannot successfully be used for titanium aluminides because of the very low ductility (brittleness) at room temperature which characterize the alloys. Consequently, titanium aluminide foil was produced heretofore by hot rolling between mild steel plates (pack rolling) which is expensive and produces a product having poor surface quality, or by chemical milling of thick plate to a foil which is expensive and wasteful.
Background information on production of titanium alloy foils is presented in "Status of Titanium Powder Metallurgy," by Eylon et al, Industrial Applications of Titanium and Zirconium: Third Conference, ASTM STP 830, pp 48-65 (1984), but the foils Produced by the methods described are not fully dense as a result of a high level of chlorides in the elemental titanium powder, and therefore have inferior mechanical properties, particularly fatigue behavior. In "Property Improvement of Low Chlorine Titanium Alloy Blended Elemental Powder Compacts by Microstructure Modification," by Eylon et al. ("Progress in Powder Metallurgy", Vol 42, pp 625-634 (1986), Proc MPIF Annual Powder Metallurgy Conference and Exhibition, May 18-21, Boston Mass. it was demonstrated that 100% density can be achieved in conventional PM compacted articles if extra low chlorine (less than 10 ppm) powder is used and if the sintered product is re-pressed, such as by hot isostatic pressing (HIP). Teachings of these references and background material presented therein are incorporated herein by reference.
The invention substantially solves or reduces in critical importance problems with previously existing methods by providing a relatively low cost method for reliably producing quality full density titanium aluminide foils of particular utility in fabricating titanium aluminide based metal matrix composite articles. According to the invention, blended irregularly shaped powder of chlorine free commercially pure (CP) elemental titanium, aluminum and other alloying metal(s) in preselected proportions are rolled into a green foil and vacuum sintered, and thereafter densified to full density by pressing such as by vacuum hot pressing (VHP), hot isostatic pressing (HIP), or additional hot rolling. Thin foils of substantially 100% density may be produced according to the invention.
It is therefore a principal object of the invention to provide a method for producing full density foils of titanium aluminide.
It is a further object of the invention to provide a method for producing full density foils of titanium aluminide in the fabrication of titanium aluminide based composites.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.